Contact electrification is the process where two dissimilar materials are touched together, and then separated, causing charged particles (electrons) to be released as bonds are broken between molecules of the two materials. A simple example of this is rubbing a balloon on your clothes so that the balloon becomes charged and is able to attract small particles or stick to walls.
The triboelectric series is used to show how well some common materials will absorb, or give up the ‘free’ electrons within them, and therefore becoming negatively or positively charged respectively. The table below shows how some materials become charged relative to each other, from most positive to most negative. The ability of the material to hold this charge is mainly dependent on the humidity of the surrounding air and the smoothness of the material. For example; a balloon will hold it charge for much longer than some fur because the individual points (the hairs) of the fur will cause corona discharge.
The Triboelectric Series
|Most Positive||Dry Human Skin||Most Positive|
|– Most Negative –||Teflon||– Most Negative –|
There are a number of devices that utilise contact electrification. A popular device is the Van De Graff Generator as it is very simple to construct. This device will produce very high voltage electricity at very low current. This means that they can be safe to touch, although it may make your hair stand on end! It works by using two rollers and a belt made from dissimilar materials. As the belt rotates, charge separation will continually occur at the point where the belt moves away from each roller. If a metal brush is placed near to these points then the charge can be collected or deposited. The choice of belt and roller materials will determine the polarity of the voltage produced.
If a sharp point (a pin) is placed on the output of a Van De Graff generator, voltage multiplier, or other High voltage DC supply, it is easily possible to witness an interesting effect. This effect is often referred to as ‘thread like streams of charged air’ Logic might suggest the ion wind should spew out in all directions (because the like charges repel each other) from the tip of the pin towards objects of opposite charge. Under the right conditions the ion wind will come out as a fine thread from the tip of the pin. This seems to contradict the idea that these charged air molecules should repel each other.
High voltage electric fields seem to be the key to unlocking some of the deepest mysteries of the universe. There are may reports of anomalies when experimenting with high voltages. For more information on the weird science, see the unexplained section.